A "molecular toolbox" for the nephrologist.

One of the many challenges facing researchers in the postgenomic era is the assignment of biologic functions to all the gene sequences now publicly available. In many centers, the mouse is the experimental model of choice for studying the genetics of human development and disease. The mouse is a powerful tool to study human biology because of the remarkable similarities of the genetic, cellular, and organ functions between these two species. Furthermore, the ability to manipulate the murine genome through targeted or random mutagenesis is unparalleled in other mammalian systems. Inactivation of genes in the germline has provided great insight into their function; however, the resulting phenotypes are often complex and may preclude analysis of the gene’s function in specific organs or tissues of interest. In part, investigators are able to overcome this problem by inactivating and/or overexpressing genes of interest in a cell-specific manner. This has required the identification and characterization of tissue-specific promoters, genetic elements that direct the expression of genes in specific cell types. Using this approach, great strides have been made in the areas of cardiac, lung, and gut development and disease (1–7). However, progress in the renal field has been slower due to the lack of useful tissue-specific promoters. In this issue of JASN, Moeller et al. (8) report the identification and characterization of two podocyte-specific promoters that promise to provide a catalyst to the study of gene function in the kidney. The authors identified regulatory regions from two genes, nphs1 and NPHS2, which are responsible for congenital Finnish nephropathy (9) and autosomal recessive steroid-resistant nephrotic syndrome (10), respectively, and are able to direct the expression of beta-galactosidase specifically to glomerular visceral epithelial cells (podocytes). In transgenic mouse models, Holzman and his colleagues elegantly demonstrate that 1.25-kb of the proximal 5' flanking region of the murine nphs1 gene and 2.5-kb of the human podocin (NPHS2) promoter contain all of the regulatory sequences required for podocytespecific expression. Wong et al. (11) have previously shown that 1.25-kb of the human NPHS1 gene also provides podocyte-specific expression, whereas larger fragments that measure 8.3-kb, 5.4-kb, or 4.125-kb direct expression to neural subsets in the developing hindbrain in addition to podocytes (12,13). In their article, Moeller et al. (8) show that only 30% of the nphs1 founder lines express the transgene, suggesting that activity of the 1.25-kb nphs1 promoter is dependent on the site of chromosomal integration. In contrast, 100% of the NPHS2 transgenic founder lines express beta-galactosidase in podocytes. The property of integration-independence is obviously important when considering which promoter to choose in generating future transgenic lines. To date, a handful of cell-specific promoters have been identified in the kidney; these include a 1542-bp fragment of the 5' flanking region of the KAP gene (kidney androgenregulated promoter) (14) and 346-bp of the gamma-Glutamyl Transpeptidase Type II promoter (15), which direct expression to the proximal tubule, 3.0-kb of the Tamm Horsfall Protein (THP) promoter, which directs expression to the thick ascending limb of the loop of Henle (TAL) and early distal convoluted tubules (16), 1.34-kb of the Ksp-cadherin promoter, which directs expression to the TAL and collecting ducts of the adult nephron and weakly in other cell types and to the ureteric bud, Wolffian duct, Mullerian duct, and developing tubules in the mesonephros and metanephros (17) while a 324-bp fragment limits expression to tubular epithelia of the developing kidney and GU tract (18), the HoxB7 promoter, which marks the ureteric bud and its derivatives (19), and 1.25-kb of the human NPHS1 and 8.3-kb, 5.4-kb, 4.125-kb, and 1.25-kb of the murine nphs1 promoters, which direct expression to podocytes (11–13). The present article adds NPHS2, another podocyte-specific promoter, to the list. How will the identification of these tissue-specific regulatory sequences help nephrologists and researchers interested in kidney biology? The most obvious answer is the ability to express genes of interest in specific cell types within the kidney and look at the resulting phenotypes. For example, increased expression of numerous growth factors has been reported to occur in podocytes during glomerular injury (20,21). Using the promoters described by Moeller et al. (8) to direct the expression of these growth factors to podocytes, it will now be possible to test whether increased expression of these factors in podocytes underlies the pathogenesis of glomerular scarring or is simply a marker of disease (i.e., do the mice that overexpress a growth factor in podocytes develop glomerular scarring?) (Figure 1). Although this is a straightforward experiment, caution must be used in interpretation of any overexpression study. Most importantly, the relevance of overexpressing a Correspondence to Susan E. Quaggin, Samuel Lunenfeld Research Institute, Room 871Q, Mt. Sinai Hospital, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada. Phone: 416-586-4800; Fax: 416-586-8588; E-mail: [email protected]